ELI5: How are the 21 Producers selected in EOS.IO blockchain?

A blockchain is created on EOS.IO and 21 Producers are selected to do Delegated Proof of Stake. How are the 21 selected?

Good overview here of Delegated Proof of Stake by Hive account@dantheman but I didn't see the answer to my question

BTW, I'm planning to post short questions like this and use the tag blockchain-eli5. Please do the same and to build up a repository of quick-fire Q&A